We propose to evaluate screening for breast and colon cancer in two cohort studies. This evaluation of the effectiveness of screening will take advantage of two ongoing prospective cohorts, the Nurses' Health Study of 121,700 women 40 to 65 years of age in 1986, and the Health Professionals Follow-up Study of 51,529 men 40 to 75 in 1986. Both cohorts are followed every two years for incident cancers and ongoing mortality surveillance includes searches of the National Death Index. In the proposed study of screening and breast cancer mortality, we will use data on screening behaviors reported in 1988 (clinical breast examination in the past two years, mammography [including time since most recent mammogram], and months of breast self-examination in the past year). We will evaluate these screening behaviors in relation to age-specific breast cancer mortality during 5 years follow-up. An additional analysis will include the stage of disease at diagnosis among the incident cases of breast cancer. We will examine the efficacy of screening within strata of risk factors for breast cancer. To determine the efficacy of sigmoidoscopy for prevention of colorectal cancer we will combine data from both cohorts, including details of past sigmoidoscopy, time since last sigmoidoscopic examination, history of polyps, family history of colon cancer, and dietary intake. Data from the Health Professionals Follow-up Study indicate that screening behaviors are not independent of diet. Hence we will examine the efficacy of screening for colon cancer within strata of dietary animal fat and dietary fiber intakes. Using data from these cohort studies we will develop a model of colon cancer incidence and mortality to estimate the number of colon cancers prevented through screening in strata of family history, age, and dietary intake.